In conventional well drilling operations such as those used for drilling oil and gas wells, the drilling fluid, sometime called ‘drilling mud’, is circulated down the well through the tubular drill pipe used to carry the bit and back to the surface to carry debris away from the drill bit. The hydrostatic pressure of the drilling fluid column also provides mechanical and physical support to the borehole wall and prevents fluid mixing between subterranean layers in the formation by providing a pressure balance.
However, as the depth of the borehole increases, the hydrostatic pressure imposed by the drilling fluid column at the bottom of the borehole increases and may surpass the fracture pressure of the formation which can lead to damage while decreasing the density of the drilling fluid to avoid this may in turn mean that it is no longer possible to provide pressure balance at shallower depths in the borehole.
In order to overcome this problem, or to finish construction of the well once the target depth has been reached, it is common to support the borehole walls by cementing a tubular liner, called a ‘casing’, into the well.
During a casing operation, a continuous casing, formed by a number of tubular sections joined end to end, is lowered into the well and cement is pumped down the inside of the casing to exit at its lower end and fill the annulus formed between the outside of the casing and the borehole wall where it is allowed to set.
Once this process has been completed, drilling can be restarted in the usual way and the cement casing provides the physical and mechanical support for the top part of the formation that was previously drilled.
Since it is necessary to maintain the borehole full of fluid during any operation, it can be necessary to prevent successive fluids from mixing as they are pumped into the well if their function is not to be compromised. For example, it may be necessary to pump cement into a well filled with drilling fluid which, if mixed with the cement, may affect its setting behaviour. Alternatively, wash fluids may precede the cement in order to remove accumulated deposits or gelled drilling fluid to improve cement performance.
One approach to dealing with this problem is to pump plugs ahead of and behind the cement to separate it from the other fluids in the borehole.
FIGS. 1 and 2 show an example of a known apparatus for deploying such cement plugs.
The apparatus comprises a tubular basket 20 that is located at one end at the end of a drill pipe 12 that is used for delivery of cement and is open at the other end. The outer diameter of basket 20 is larger than the drill pipe and houses cementing plugs 18, 22. As can be seen, each plug has an opening at its lower end that is smaller than the inner diameter of the drill pipe 12. The opening of the lower plug 22 is in turn smaller than the corresponding opening in the plug above 18.
The openings in the cement plugs 18, 22 allow for fluid flow therethrough. However, the limited size of the openings is a limiting factor for pumping fluids at a high flow rate. Additionally there is a risk of eroding the inner diameter of the cement plugs when pumping the solids laden fluids through the plugs.
Deployment of the plugs 18, 22 is achieved by pumping darts 10, 14 from the surface through the drill pipe 12. The darts 10, 14 seat in the openings of the plugs 18, 22 so as to block fluid flow and the pressure caused by this blockage causes the plug to be driven from the basket 20 (see FIG. 2). The bottom dart 14 has a smaller profile than that of upper dart 10 in order to be able to pass through opening in the upper cement plug 18 to seat in the opening of the lower plug 22. As a result, it is vital that the cement plugs are loaded into the basket 20 in the correct order and that the darts 10, 14 are launched down the drill pipe 12 in the correct order so that the correct cement plug is injected into the well.
The number of cement plugs that can be loaded into the basket is limited due to the need for a constantly decreasing inner diameter of the opening in each consecutive cement plug. In the current state of the art, two cement plugs per basket is the limit.
It is therefore the object of this invention to provide a new apparatus for injecting cement plugs into a well which protects the inner diameter of the cement plugs from being eroded by fluid being pumped through them, allows for more than two cement plugs to be stored and injected into the well in one trip.